It has long been a desire of various environmental scientists, organizations and agencies to be able to complete effective studies or research relating to erosion and pesticide runoff such as occurs on agricultural fields or turf. For example, field studies designed to quantitate pesticide runoff and subsequent contamination of surface water need to be performed to obtain aquatic exposure assessments. Experience has, however, shown that pesticide runoff studies driven by natural rainfall are sometimes unsuccessful because the intense rainstorms typically required to provide useful data do not occur during the limited period of time when high pesticide residue levels remain in the field. The inherent risk associated with the weather dependent nature of these studies and the high cost of conducting experiments at multiple locations as needed to describe the runoff on a regional basis has led to the development of rainfall simulators adapted to complete the studies in a quick and cost effective manner.
In order to complete a valid study accurately reflecting what is naturally occurring in the environment, it must be appreciated that natural rainfall and, more particularly, natural rainfall as found in the particular geographic region under study needs to be simulated as closely as possible. This allows a "designed" rainstorm to be generated at any time after the pesticide is applied, thereby ensuring that the necessary weather conditions exist for a constructive study.
Attempts to accurately simulate natural rainfall began at least as early as the 1930's. The attempts to date have, however, met with only limited success. This is because the duplication of the rainfall characteristics of a natural storm is extremely difficult. In some instances the rainfall characteristics of geographical regions are not known in sufficient detail to allow accurate descriptions of the storms. In others, while the rainfall characteristics are known, it has been impossible up to the present time to develop a device capable of reproducing all desired characteristics. Accordingly, up to the present time complete rainfall simulation has not been possible. Accordingly, it has also not been possible to undertake completely accurate studies of erosion and pesticide runoff on anything other than small research plots of perhaps a few square meters.
A large number of natural rainfall characteristics or parameters must be considered in order to provide an effective simulation. This is particularly true as research has not as of yet clearly established the relative importance of the characteristics. Some of the characteristics that must be considered include: (1) the droplet size spectrum; (2) the droplet impact velocity with the ground; (3) the intensity/duration of the rainfall application; (4) the spatial uniformity of the droplet distribution over the test plot; and (5) the continuous application of the simulated rainfall over the entire test plot. Additionally, the expense of constructing, maintaining and operating a system must also be considered. Further, the portability of the system is important as on-site studies are absolutely essential and many sites are found in remote areas. It is also desirable to provide a simulation apparatus that may be quickly and efficiently reconfigured to simulate different natural rainfall such as may occur during a different season, in a different type of storm and/or in a storm from another geographic region.
One of the better rainfall simulators developed to date is disclosed in the article entitled "Multiple-Intensity Rainfall Simulator for Erosion Research on Row Side Slopes" by L. D. Meyer and W. C. Harmon reprinted from the transactions of the ASAE, 1979. This simulator includes oscillating spray nozzles to apply droplets of a size and at an impact velocity closely approximating natural rainfall.
Despite effectively simulating these two parameters, the device suffers from a number of shortcomings. More particularly, the oscillating spray nozzles only intermittently apply simulated rainfall over the test plot. This results in a non-uniform input of water and energy to the test plot over time unlike that associated with natural rainfall. The application of intermittent "rainfall" may significantly effect erosion patterns and pesticide runoff resulting in skewed data that lead directly to inaccurate conclusions.
It must also be appreciated that the design of the rainfall simulator in question only allows the application of the simulated rainfall over a relatively small test plot. The relatively small size of that test plot does not allow the collection of data that may be reasonably extrapolated to field-scale or meso-scale conditions. More particularly, a small test plot cannot support typical agricultural practices with respect to tillage and application using common farm equipment. This "tillage effect" is particularly significant because much of the water moving over a field is often conducted along the compacted tracks formed by modern farm machinery.
Further, while it is possible that a number of the apparatus in question could be positioned adjacent each other in a field to effectively provide a larger test plot, the viability of such a system is questionable. More particularly, the system would be very expensive to produce and operate. It would also be difficult to transport, arrange and effectively operate under field scale conditions. Further, the combined system would still rely upon oscillating spray nozzles for "rainfall" application. Thus, the drawbacks of intermittent energy input are not overcome. Accordingly, it should be appreciated that a need very clearly exists for an improved meso-scale or field-scale rainfall simulating apparatus particularly adapted to provide better simulations and accordingly, more accurate research data upon which to base study conclusions.